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Abstract

In many potential applications of two-dimensional (2D) electronic spectroscopy the excitation energies per pulse are strictly limited, while the samples are strongly scattering. We demonstrate a technique, based on double-modulation of incident laser beams with mechanical choppers, which can be implemented in almost any non-collinear four wave mixing scheme including 2D spectroscopy setup. The technique virtually eliminates artifacts or “ghost” signals in 2D spectra, which arise due to scattering and accumulation of long-lived species. To illustrate the advantages of the technique, we show a comparison of porphyrin J-aggregate 2D spectra obtained with different methods following by discussion.

Figures (3)

Schematics of the experimental setup (distances are not to scale). Two parallel beams of femtosecond laser pulses (1&2 and 3&4) are focused by a spherical mirror SM1 onto a transmission diffractive grating G. Only the first positive and negative diffractive orders are generated with high efficiency, the rest is blocked by a mask (not shown). The resulting four beams are directed to a spherical mirror SM2 and focused into the sample S. Beam 4 (LO) is attenuated by the neutral density filter F (OD = 3). The time delays of beams 1, 2, and 3 are finely adjusted by movable fused silica wedge pairs W1, W2, and W3, respectively. W1 and W2 are scanned during the measurement, while W3 is kept constant. Beams 1 and 2 are modulated by choppers C1 and C2, respectively. The emitted third order signal field and LO propagate in the same direction through the iris I, while beams 1, 2, and 3 are blocked. M1 and M2 are planar folding mirrors. Inset shows the timing diagram of the pulse sequence used.

Data acquisition and processing sequence (schematics). For clarity, all the data except of chart (f) is displayed for a single modulation case. (a) A strip of pixels of the CCD sensor (3-5 rows) is illuminated by interfering signal and LO beams. (b) Hardware binning is applied: groups of n CCD rows are shifted into the register and read-out resulting in single lines in the acquired data set (c). A spike rejection and a high-pass filters are applied to the columns of the acquired data set: (d) – a single column before filtering, (e) – the same column after filtering. (f) The view of the filtered data in the frequency domain (a double modulation case, see text for details). (g) An interferogram extracted by calculating the Fourier coefficients at the signal modulation frequency: fmod for the single modulation case, |fmod1-fmod2| and/or fmod1 + fmod2 for the double modulation case.